The present invention pertains to gas turbofan engines and, more particularly, to actuating mechanisms for variable pitch fan blades used therein.
Jet engines for powering aircraft obtain subsonic performance improvements by use of a fan. The fan is rotatably driven, through shaft connection, by the turbine portion of the engine and serves to pass a large volume of air around the engine, thereby increasing overall engine thrust. In fact, in recent typical commercial fan engines, the fan moves several times as much air as is taken in by the engine compressor. Recently, extensive work has been directed toward varying the pitch of the fan blades. It is believed that many of the same advantages can be obtained as are available with variable pitch propellers since fans are, in reality, directly analogous to propellers with shrouds.
Accordingly, the variable pitch fan is being considered for application on high bypass turbofan engines for improved performance in several areas: noise reduction, thrust reversal, improved thrust response, and overall engine performance. For instance, fully reversible variable pitch fans have been proposed for high bypass ratio engines on STOL-type (short take-off and landing) transports. Cascade-type and target-type thrust reversers which change the direction of the fan airflow passing around the engine (the bypass stream) become large and heavy so that it is highly desirable to find alternative means for generating decelerating thrust. It is possible to generate substantial reverse thrust in a high bypass ratio fan by fan blade pitch reversal which induces pumping of the airflow through the fan blades in the reverse direction.
Recognizing the inherent advantages of incorporating a variable pitch fan in a gas turbofan engine, it becomes necessary to develop a mechanism for efficiently and effectively producing fan blade pitch changes. Consideration must also be given to providing a means of arresting the mechanism to avoid possible engine damage should a malfunction occur which would cause a run-away control.